This thread is only on lactic acid. There is much more to saving a fish that just, "get it in fast". Barbless hooks, slim coat, ect... all play a role.

The Easy answer-
I once heard from a guide that smaller fish are much less fragile than larger fish. He was a retired biologist; therefore, I can't agrue with that. It seems the fragility of fish's osteology increases as it grows larger when it is taken out of water.

If you have a good size fish on and the current is strong, but the fish is tiring down and you still don't feel you can get him in yet. If the fish swims up river and as you pull it back you see it doing a half backflip, get it in! Forgete the tippet, just save the fish.

The more indepth answer-

Most all of us have heard of lactic acid build up, but do you really know what happens? Lactic acid build up is probably the #1 reason for fish not surviving a long fight. Large amounts of pyruvic acids are formed when the fish is fighting (from glycolysis). When there is not enough oxygen to break down those carbs and get those ATPs (energy from a phosphate group being broke of) rolling it will start making lactic acid. The fish only has a certain lactate threshold, and once you pass this point you risk killing that beautiful wild stealhead (or other fish) because it won't be able to recuperate from the build up of lactic acid. The lactic acid will enter the blood stream (acid = low Ph (below 7) = bad). Out blood Ph is around 7.4 and a little inbalance can cause havoc in the body. I would think the same goes for fish.

I am not a biologist so I am probably not 100% correct. I just thought I would share a little knowledge from my much enjoyed Biology class.

That was very good, and I believe you said this in a manner that readers will understand. With years to practice I've gotten real good at landing even large salmon and steelhead trout fast, and I mean fast. I am not a guy who enjoys the 'fight', the fight is when you lose the fish. If you want to catch them then you gotta do just that and let em go just as quick. Almost all fish pictures I have fall into 2 categories; live trout, char, salmon, or grayling in the water, or dead salmon that we intend to eat.

Yeah those are the times I only keep fish is when I believe they wont make a recovery, otherwise it's all catch and release.
I don't even eat fish but once or twice a year so there is not much demand for it at my kitchen table.
I asked here earlier in the year about a camera for taking pics as all the members here have such great shots, but my realization is that I am just too uncoordinated with a camera to be able to take those shots in any decent amount of time as to not harm the fish. So I think I'll forgo that, you guys will just have to take my word on it when I tell you I caught a 28" out of the Neversink.
Thanks for the explanation of what goes on in the fishes body when it is under stress, I too am not a biologist and do not read into technical papers to well, this was easy to understand.

This might be right in theory but the problem with is is that is how it works for mammals, specifically for humans. If it were the same the fish would be through its anaerobic energy systems within the first thirty seconds when all the ATP and phosphocreatine were used up. The main problem is that fish muscle phys is incredibly different from ours. When ours contract it generates heat, a fishes doesn't so i can't say how it works exactly and you might be right but I think there is a pot more to it than a fish exceeding.g its lactate threshold.

Aerobic metabolism (requires oxygen) is much more efficient than anaerobic (does not require oxygen). So when oxygen in the blood drops below a certain level, metabolism switches to anaerobic. The end product of anaerobic metabolism is lactic acid and too much creates acidosis (too low a pH).
Taking a fish out of the water will also kill it, especially as it becomes larger. The vertebrae are not designed to support weight as they are in land animals. Holding a large fish under the head and the tail causes the body to sag and will most likely cause a "broken back" or at least damage the spine.
Alaska, I believe, requires you to keep any fish that is removed from the water.
Is that correct, Ard?

While build up of lactic acid is a major cause and does contributes to fish mortality it's not the direct cause in most cases. Before we get to the real bugger in killing fish, let's look at a few specifics that apply:

(1) First the amount of handling, fighting and time out of water varies tremendously from species to species. Some catfish can take several minutes of air time and survive quite nicely while a Rainbow might not fare well after just 30 seconds of air time as research shows that to be at nearly 30% mortality if everything else was perfect except for the fish being out of water for the 30 seconds.

(2) Water temps and the fish's normal range of preferred temperatures plays a huge part in fish kill. If the fish is accustomed and prefers water at 58 degree F and you catch and release it within 30 secs, it will have a much better chance for survival than one caught in 68 degree water. In short, water that's a bit too warm, holds less oxygen and the fish suffer when held out of the water on top of fighting in that warm water.

(3) Hook and hooking mortality plays a huge part. Hooks that are larger cause more damage, multiple barb hooks cause more damage, hooks with barbs cause more damage, hooks or flies that lend themselves to swallowing cause more damage. A hook in the gill area causes severe damage. In short use barbless hooks that are appropriately sized and don't allow fish to swallow the fly if you can help it.

(4) Predators can chase down and kill an exhausted fish quickly and do. Play the fish quickly get it in, even put it in a live well, till the coast is clear and then release. This usually applies to saltwater fish but I suppose it happens in freshwater also.

(5) Handling fish improperly causes mortality to shoot upward. Big heavy fish should only be handled with both hands, one on the head and one supporting the belly, as a one hand, jaw lift usually causes damage. Trout and others in the salmonid family should be handled as little as humanly possible as they lose slime easily to hands and are very susceptible to injures from fungus after the slime is gone and from pressure damage to internal organs as they are slippery and folks tend to squeeze a bit too much. Use a hook release if at all possible and release in the water, and for trout the Runje's Releaser, available from Silver Bow in Spokane where it was invented is inexpensive, very simple, and the best I've found.

(S) to summarize the bad boy of the above bunch is suffocating the fish by keeping it out of water too long. Warmer water demands a fast fight and quick, in water, release if the trout is to live. High mid summer temps in some trout streams demand no fishing if you want the fish to live and all of this talk about temps and even lactic acid above, directly relates to the way the fish takes in oxygen and then uses it in addition to how water hold oxygen and what part temperature plays there.

After all that please keep in mind that an automatic catch and release is not always best for the fish and fishery. I prefer the Selective Release that the In Fisherman crowd coined. Too many brookies in a western stream--catch and keep! Browns or rainboews threatening an eastern Brook Trout stream, catch and keep! Ample fish and fish survival such as often is found with bluegills and with some runs of salmon -- catch and keep! When the fish is an invasive species threatening native species it edges upward on the catch and keep ladder! Prolific and under fished species such as Bluegill, Perch, Sunfish, Perch, and Crappie in many places, catch and keep! In short Selective Release demands a better than average knowledge of the fish and the fishery and lends itself to conservation!

" Browns or rainboews threatening an eastern Brook Trout stream, catch and keep! Ample fish and fish survival such as often is found with bluegills and with some runs of salmon -- catch and keep! When the fish is an invasive species threatening native species it edges upward on the catch and keep ladder!"

I once heard from a guide that smaller fish are much less fragile than larger fish. He was a retired biologist; therefore, I can't agrue with that. It seems the fragility of fish's osteology increases as it grows larger when it is taken out of water.

I think a big part of what he was referring to pertains to the differences in gravity between in the water and out. The ligaments and other connective tissue that suspends a fish's organs within its body don't need to be very strong, since the fish is submerged in the water all the time. Once their pulled out of the water, there's MUCH more gravity pulling down on their stomachs/egg sacs, etc.

Its pretty obvious that gravity is wreaking havoc on these fish's internal organs-

And there's just NO WAY a fish's jaw is designed to support its full weight like this-

Chuck, very good information. Those topics you talked about are the main things people should remember when dealing with the fish. The biology and geography of fly fishing is why I fly fish, mainly...More importantly we need to remember to respect the river as a hole, more than just the fish.